Electrolytic device and semiconductor oxide electrolyte therefore

ABSTRACT

A solid electrolytic capacitor is produced by competing electrolytic reactions that produce a dielectric film on the surfaces of a porous anode and a layer of semiconductive oxides on the dielectric film. Current is passed through the anode and a suitable cathode in an aqueous electrolyte containing a filmforming anion and cation capable of being oxidized to a semiconductive layer on the surface of the dielectric film. The thickness of the semiconductive layer is controlled by an intermediate step of depositing a given amount of a hydroxide of the cation on the dielectric film by reversing the current between the initial and final oxidation.

United States Patent [191 [111 3,732,470 Robinson 1 May 8, 1973 [541ELECTROLYTIC DEVICE AND 3,254,390 6/l966 Shtasel ..29 25.31 S CO O OXIDE3,345,544 10/1967 Metcalfe ..3l7/230 3,397,446 8/1968 Sharp ..29/570ELECTROLYTE THEREFORE 3,631,302 12/197] Robinson ..3l7/230 [76]Inventor: Preston Robinson, Bulkley St., Williamstown, Mass 01267Primary ExaminerJames D. Kallam 22 Filed: Dec. 20, 1971 [57] ABSTRACT PP210,083 A solid electrolytic capacitor is produced by compet- RelatedU.S. Application Data Continuation-impart of Ser. No. 767,516, Oct. 14,1968, Pat. No. 3,631,302.

References Cited UNITED STATES PATENTS 8/1963 Sherman .;317 230 x ingelectrolytic reactions that produce a dielectric film on the surfaces ofa porous anode and a layer of semiconductive oxides on the dielectricfilm. Current is passed through the anode and a suitable cathode in anaqueous electrolyte containing a film-forming anion and cation capableof being oxidized to a semiconductive layer on the surface of thedielectric film. The. thickness of the semiconductive layer iscontrolled by an'intermediate step of depositing a given amount of ahydroxide of the cation on the dielectric film by reversing the currentbetween the initial and final oxidation.

4 Claims, 2 Drawing Figures 121 17210 flm gq l i 14 DieZecZrz'cOxidelfiSemz'cozzdach've Oxide I J8Ca'lkod'e Cbmeai'or ELECTROLYTIC DEVICEAND SEMICONDUCTOR OXIDE ELECTROLYTE THEREFORE CROSS-REFERENCE TO RELATEDAPPLICATION BACKGROUND OF THE INVENTION This invention relates to anelectrolytic device and a semiconductor oxide electrolyte therefore, andmore particularly relates to a solid electrolytic capacitor employing insitu formed oxides.

My above-identified parent application discloses the preparation of asolid electrolytic capacitor in an electrolyte containing a film-forminganion and a metal cation capable of being oxidized to a semiconductiveoxide and wherein the two electrolytic oxidations take place atdifferent speeds. the faster oxidation generally being the formation ofthe dielectric film at constant current, wherein the voltage across theelectrolytic cell increases steadily with concommitant formation of thedielectric film until a critical voltage is reached, -at which point thevoltage remains steady for a length of time depending on theconcentration of the metal cation in the electrolyte and thesemiconductive layer is formed over and in intimate contact with thedielectric layer, whereupon the voltage again begins to risecorresponding to renewed formation of the dielectric layer. At aperdetermined voltage, the voltage is maintained at a fixed value andthe current decreases until an arbitrary value is reached. This is theresult of formation of dielectric film both by electrolytic oxidationand by the self-healing interaction of the semiconductor layer withunreacted metal below pores in the films as evidenced by random sparkingand momentary increases in the leakage current followed a greaterdecrease in the leakage current.

The anode is then ready to be made into a condenser by the formation ofa cathode usually by coating with colloidal graphite, drying, andfurther coating with. a metal such as silver or copper/As noted in myabove identified parent application, the semiconductor layer so formedis sometimes too thin to stand up under the mechanical handling ofmaking the cathode and this layer must then be thickened.

It is an object of this invention to provide an electrolytic devicehaving oxidesformed in a continuous in situ process that are thickenough to withstand subsequent manufacturing procedures.

Another object is to provide a continuous in situ production of oxidesin and on a porous pellet.

SUMMARY OF THE INVENTION- In accordance with this invention the in situproduction of oxides on an anode body by continuous anodization in anelectrolyte containing both a film-forming anion and a semiconductiveoxide forming cation includes thickening the oxides at a suitable stagein the process by reversing the current whereby the hydroxide of thecation is deposited on what is now the cathode and when the current isagain reversed the hydroxide is oxidized to form additional thickness tothe semiconductor layer.

I have found this process to be of particular importance in the case ofporous anodes. Here the formation of the dielectric film takes place onall the surfaces of the anode exposed to the electrolyte, but thesemiconductor layer is formed first of the outermost layer of the anodeand tends to interfere with the continued formation of thesemiconductive layer in the innermost recesses of the porous anode.However, by

reversing the current, the cations are carried into these innermostrecesses and are precipate d as the hydroxide. On again reversing thecurrent, the hydroxide is oxidized to form the semiconductive layer onthe dielectric film throughout the porous anode.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a cross section of acondenser made of a porous anode in accordance with this invention; and

FIG. 2 is a flow sheet of the method of this invention by which thecondenser of FIG. 1 is produced.

DESCRIPTION OF THE PREFERRED EMBODIMENTS vided over the interior coatingof semiconductive oxide 16.

While porous tantalum anodes made by various powder metallurgytechniques are commercially available, those of aluminum are not. The Alanodes used herein were made of 99.99 percent Al powder through 320 meshor.400 mesh. A weighed amount of powder was placed in a cylindricalcavity with movable pistons at both ends. One piston was perforated topermitinsertion of a lead-,wire into the powder. Both pistons were thendisplaced a predetermined amount with pressures of about 250-300 psi andproduced a pellet of the desired porosity. The pellet was then sinteredat near 600C in a relatively inert atmosphere of- He or A or in a vacuumand produced pellets ranging from 50 to 55 percent of bulk density ofAl, thus with a porosity of 50 to 45 percent. In some cases the exterior.of the pellet had a thin brownish superficial coating when sintered invacua of 10- torr, by reacting with residual N However, the formation ofa dielectric oxide coating was accomplished with no increase in powerconsumption and with a disappearance of the color.

The color could also be removed by making the pellet the cathode in anelectrolyte containing a strong alkali cation such as Na, K",quanidine", or quaternary ammonium ions or the like. While thistreatment is unnecessary to remove the brown or black stain, it had thedesirable effect of dissolving or dislodging any unsintered aluminumpowder left in the pellet. This is desirable and is thereforeincorporated into one of my process steps which deposits a metalhydroxide in the pores of the pellet.

An electrolyte is made up containing 8 grams per liter ammoniumdihydrogen phosphate and 8 grams per liter of lead acetate. A fine whiteprecipitate of a lead salt is formed which is maintained in suspensionby suitable agitation and the resulting electrolyte has a pH in theregion of to 5.5. A porous tantalum anode weighing 0.75 grams is madethe anode in this electrolyte, and a suitable cathode which may becarbon is provided. On'pa'ssing a direct current of I00 milliamperesbetween the electrodes, the voltage increases steadily in 9 or 10minutes to a voltage in the neighborhood of 175 to 200 volts and this isthe formation of a dielectric layer. At this point the formation of thesemiconductor layer begins and at a current of 100 milliamperes thevoltage drops to the neighborhood of 125 volts and over a period of 10minutes increases again to the former voltage of between 175 and 200volts and this is primarily the formation of the semiconductor layer.Subsequently at the same current, the voltage rises more rapidly to theneighborhood of 300 volts which is close to what is known in the art asthe breakdown voltage of the electrolyte. When this voltage ismaintained constantly for a few minutes, the current drops rapidly from100 milliarnperes to below 10 milliamperes.

The electrodes from the foregoing example are then made into condensersby adding suitable cathode material such as suspension of graphite(aquadag) followed by the application of a conductor which may be silverin the form of paste or other conductor provided by flame spraying orsputtering. The assembly is then suitably dried to remove the lasttraces of solvent employed in the various processes and assembled in asuitable container. The anode from Example 1 has the followingcharacteristics: a leakage current of 1 milliampere and a capacity of5.0 microfarads.

While these condensers have utility, it is desirable in some cases toemploy a thicker semiconductor layer because of the rough handling whichmay result inassembly operations. This may be accomplished by em ployingelectrolytes containing higher concentrations of the cation whichgoes toform the semiconductor oxide layer, and by reversing the current toproduce metal hydroxide in the pores of the anode, and then reoxidizingin the original electrolyte.

EXAMPLE 2 A solution of 8 grams per liter of lead acetate is neutralizedwith alkali until a slight permanent precipitate is formed. A Taelectrode as used above, is made the cathode in this bath, a suitableanode is carbon, and a current of 100 milliarnperes is passed for oneminute. The Ta electrode is now coated with lead hydroxide is thensubjected to the process of Example 1 giving characteristics of 350microamperes and a capacity of 8.5 microfarads.

It will be observed that this treatment has nearly doubled the capacityof Example 1 and developed more than 90 percent of the capacity measuredin an aqueous electrolyte. Evidently, the semi-conducting layer inExample I plugged some pores in the anode preventing access to all thedielectric film, and that by carrying out the process step of Example 2,nearly the entire capacity was realized, together with a decrease inleakage current.

EXAMPLE 3 A smaller Ta anode of 0.55 gms was anodized in an aqueouselectrolyte containing 7 g.p.l. of H C10 and 8 g.p.l of Pb (C H C towhich sufficient (C H N was added to bring the pH to 6. Dielectric filmwas formed to about V whereupon the PbO- was formed. The electrode wasthen made the cathode in a boiling electrolyte of 2M Pb(NO and 0.3M KNOand 50 milliarnperes was passed for 30 seconds, depositing Pb(OH)- inthe electrode and evolving H The electrode was then made the anode inthe original electrolyte and reanodized. The oxidation of Pb(OH) to Pb0began at about 60 V and was completed at V at which voltage the leakagecurrent dropped rapidly. The electrode was rinsed thoroughly indistilled water and given a final formation in an electrolyte l g.p.l.of trimellitic acid until the leakage current was less than 1milliampere. The condenser was finished as in Example 1 and showed acapacity of 1.5 mfd. and a series resistance under 100 ohms.

Similar results were obtained when Ni and Mn were substituted for Pb inthe boiling nitrate reverse current step, the Ni giving identical resultwhile the Mn gave a lower capacity and a higher resistance due to thepoorer conductivity of MnO Some care must be exercized in the reversecurrent step lest the pores be completely filled with the semiconductiveoxide and the characteristics of the device become governed by thislayer. Thus doubling the coulombs passed in Example 3 cuts the effectivecapacity to 0.50 mfd., and passing 10 times the coulombs cuts it to0.025. Thus there is an optimum value of the coulombs readily determinedby those skilled in the art.

EXAMPLE 4 An Al anode described above weighing 0.4 gms was anodized inan electrolyte of l g.p.l. trimellitic acid to.

50 V forming a dielectric film. The electrode was made a cathode in aboiling electrolyte of 2M Pb(NO 0.3M KNO and 0.00lM Ag C H O and 50milliarnperes was passed for 10 seconds. The electrode was rinsed indistilled water and reformed to 100 V in the original electrolyte givinga capacity of 10.0 mfd. and a series resistance under 10 ohms. 1

For film forming metals l have used Ta, Al and Nb. For cations formingsemiconductive layers l have used Pb, Mn, Ni, Ag and mixtures thereof.For film forming electrolytes, I have used the following acids and theirsalts giving pI-ls between 4.5 and 8.5: trimellitic acid, pyromelliticacid, terephthalic isophthalic, uric, sulfamic, perchloric, persulphate,phosphoric. l have avoided those combinations which produce salts ofextremely low solubility such as lead pyromellitate.

In the reverse current step when hydroxides are I formed by thegeneration of hydroxyl ions at the cathode, l have found it preferableto start with acid solutions of pH6 or lower as otherwise the hydroxideprecipitates on the outside of the electrode rather than in the pores.

I have also found that with strong oxidizing acids such as perchloricand persulfate that the semiconductive oxide can be produced chemicallyas well as electrochemically. This is of no great harm but it doesconsume the cations and they must be continually replaced. This purelychemical reaction may take place preferentially in the porous electrodeby the prior introduction into the electrode of Ag as a catalyst.

In the case of Al, I have found some dielectric oxide film must bepresent if chemical reduction of the cations and resulting corrosion ofthe Al is to be avoided.

What is claimed is:

1. An electrolytic capacitor comprising a porous pellet of film-formingmetal, a dielectric oxide film formed on the surfaces of said porouspellet, a reducible semiconductive oxide electrolyte in intimate contactwith said dielectric film, a portion of said semiconductive oxide beingconverted from the hydroxide, and a conductive cathode connector incontact with said oxide electrolyte, said dielectric film and saidsemiconductive oxide electrolyte being in situ formed in one continuouselectrolytic formation of said porous pellet in a single solutioncontaining both a film forming anion and a semiconductive oxide formingcation.

2. The device of claim 1 wherein said porous pellet is a member of thegroup consisting of aluminum, tantalum and niobium; said film forminganion is a member of the group consisting of phosphate, borate,persulfate, urate, trimellitate, pyromellitate, terephthalate,isophthalate, perchlorate and sulfamate; and said semiconductive oxideforming cation is a member of the group consisting of Mn, Ni, Ag, Pb,and mixtures thereof.

3. A method of making an electrolytic device having a film-forming metalcoated with a dielectric film and a superimposed semiconductive layercomprising the steps of producing a porous pellet of a film-formingmetal, anodizing said porous pellet in an electrolyte containing both afilm forming anion and a semiconductive oxide forming cation to producesaid dielectric film, cathodically depositing the hydroxide of saidcation in and on said porous pellet, continuing anodization in saidelectrolyte to produce said semiconducting oxide layer in intimatecontact with said dielectric film, continuing anodization in saidelectrolyte still further to produce additional dielectric film, andmaking con nections to said porous pellet and to said semiconductingoxide layer.

4. The method of claim 3 where a cation of a strong alkali is alsopresent. I

2. The device of claim 1 wherein said porous pellet is a member of thegroup consisting of aluminum, tantalum and niobium; said film forminganion is a member of the group consisting of phosphate, borate,persulfate, urate, trimellitate, pyromellitate, terephthalate,isophthalate, perchlorate and sulfamate; and said semiconductive oxideforming cation is a member of the group consisting of Mn, Ni, Ag, Pb,and mixtures thereof.
 3. A method of making an electrolytic devicehaving a film-forming metal coated with a dielectric film and asuperimposed semiconductiVe layer comprising the steps of producing aporous pellet of a film-forming metal, anodizing said porous pellet inan electrolyte containing both a film forming anion and a semiconductiveoxide forming cation to produce said dielectric film, cathodicallydepositing the hydroxide of said cation in and on said porous pellet,continuing anodization in said electrolyte to produce saidsemiconducting oxide layer in intimate contact with said dielectricfilm, continuing anodization in said electrolyte still further toproduce additional dielectric film, and making connections to saidporous pellet and to said semiconducting oxide layer.
 4. The method ofclaim 3 where a cation of a strong alkali is also present.